DNA mismatch repair (MMR) is the major pathway of mutation avoidance in all organisms. Defects in human MMR genes have been directly linked to the development of cancer including hereditary nonpolyposis colorectal cancer. The striking sequence homology between bacterial and eukaryotic MMR proteins indicates that exploring bacterial MMR will advance our understanding of the regulation of mutation avoidance, genome stability and cancer development. Bacillus subtilis, a well-developed system for protein localization experiments, will be used as a model organism for MMR because like eukaryotes it does not employ dam-directed methylation for DNA strand discrimination. This work will characterize the localization of the B. subtilis MMR proteins MutS and MutL to DNA mismatches in vivo. I will monitor the localization of wild type and mutant MutS fused to green fluorescent protein (GFP). I will examine the interaction of MutS mutant proteins with the bacterial processivity factor, the beta subunit of DNA polymerase III, and their participation in strand discrimination. Other experiments will seek to identify accessory proteins that participate in MMR. This work will reveal the careful orchestration of MMR within the framework of a living cell.